As Rick said, what size do you want to bale? When I was looking at haying my dealer recommended the smaller Vermeer Rebel baler to go behind my Boomer 8N. It will do 4'X5' bales with a minimum of 40HP, 50HP recommended. The larger size will do 5'X5' bales with 50HP minimum, 60HP recommended. My Boomer 8N only has 40HP PTO so I held off as I didn't want to do minimums.
I am looking at 75 HP tractors now such as the NH Workmaster 75 to use a 10 square bale hay grapple. With ballast I will be around 7000# so it should handle my needs. I will be look at haying options again, most likely the smaller Vermeer Rebel.